This paper aims to follow the history of the principle of subsidiarity to define it as a directive concerning the form of government. The roots of the principle may be found in antiquity; however, it was the medieval period when, despite not having been articulated, its essence was visible both in political and legal practice and in philosophical thought, especially in Thomas Aquinas. However, in the subsequent period, rising doctrines of sovereignty and social contract, introduced by William Ockham and Marsilius of Padua, developed later by Jean Bodin, Thomas Hobbes, John Locke, and Jean-Jacques Rousseau caused much centralization of political power during the modernity and therefore reduced the application of the principle of subsidiarity. Nevertheless, the theoretical development of the principle was continued by Johannes Althusius at the turn of the 16th and 17th centuries and, in the 19th century, by Alexis de Tocqueville and finally by Catholic social teaching, with which it got its mature form and name. A review of this history allows the author to reconstruct the essence of the principle, which may be described as the necessity of the existence of political beings smaller than central government whose autonomy may not be infringed.